Nope. Nuclear thermal propulsion has a high specific impulse ISP (roughly double that of the best chemical rockets driven by hydrogen/oxygen) but a low thrust, too low to get off the pad. They must be launched by chemical rockets and would then propel a spacecraft on its transfer to Mars. There is no environmental gain unfortunately for the atmospheric part.
When something goes wrong at launch with an NTR motor on bord and the reactor breaks open on impact that'll be more or less bad, depending on how the nuclear fuel is stored and who finds it first and does what with it. For a taste of a bad case search Kosmos 954, though there are techniques to avoid such a scenario. Nasa has studies for the risks involved with a launch of the probes that have radioisotopic generators on bord. It is not super bad, but not exactly in the comfort zone either.
Plasma thrusters, like the ion thrusters belong to the family of electric propulsion, have a too low thrust for a manned spacecraft. They are among the most effective propulsion methods (2-40 times the ISP even of an NTR) that exist with current tech, and used for spacecarft who have years to decades time, use multiple gravity assists, etc. They need, though a lot of electric energy, making them almost useless beyond Jupiter if driven by solar panels.
For the significance of the ISP let me point to Tsiolkovski's rocket equation.